1. Field of the Invention
There is an increasing need for devices to alert the police or other agencies to the entrance of an intruder or the disturbance of containers, covers, doors, etc. Some of these devices are designed to give an audible signal when there is a disturbance. Others are designed to transmit the information to a remote location without giving an indication at the sight that an alarm has been given. Sometimes a combination of local alarm signals and remote signals are given.
The increased cost of fuel supplies, and shortages, has increased the need for devices which will render fuel containers more secure. The cost of gasoline has risen to the point that thievery has increased. In the past such has been mainly confined to stealing of small quantities such as that which might fill a vehicle gas tank, but now larger quantities are being taken. Gasoline stations that are closed, such as on Sunday, or even for longer periods are particularly apt to be the subject of professional thieves. In the ground tanks at gas stations can contain substantial quantities of fuel. A tank truck can be easily driven into a station, the manhole cover removed, the fuel pipe opened and gasoline removed. It is normal to see a tank truck at gasoline stations. Ninety-nine percent of the time if they are delivering fuel. On the rarer occassions when a station is being permanently closed or new tanks are to be installed the trucks are used in removing fuel from the tanks. The average person seeing a tank truck parked at a gasoline station would normally assume that a delivery is being made. Even when a gas station is closed, and gasoline is being removed the average person would not likely assume that the removal was unauthorized.
The underground tanks are served by filler pipes covered by a manhole cover. The filler pipe extends downwardly from beneath the concrete or other surfacing of the station to the in ground tank. A cylindrical metal housing usually extends through the concrete or like surfacing to the filler pipe. The housing is closed by the manhole cover. The manhole cover simply rests in place in the housing. It is removed by prying upwardly.
The covers can readily be removed by almost anyone and therefore it's relatively easy to gain entrance to the underground tank fill pipes. With a proper length of hose and a pump, one could fill his tank truck readily from the underground supply.
It is thus desirable to provide some means for signaling when the manhole cover is removed.
2. Description of the Prior Art
It is known to provide alarm devices which will alert anyone near by to the possibility of unauthorized entry by an audible alarm. One such device is illustrated in U.S. Pat. No. 3,831,163 issued in 1974 to Byers. This device will detect tilting of a vehicle. Tilting might also occur when a manhole cover is removed to provide access to a filler pipe. The use of a radio transmitter and receiver is also mentioned in the Byers' Patent as a means of giving a signal at a remote location.
The inventor in U.S. Pat. No. 3,831,163 uses convex and concave conductive diaphrams bridged by a ball. The ball will move along one of the diaphrams if the unit is tilted and when it reaches a certain position it will bridge the space between the two diaphrams and an electrical circuit will be completed. In another form of the patented invention a conducting ring is provided between the diaphrams and the ball which will make contact with the ring when the device is tilted thus closing the circuit.
There are also many well known devices which will trigger an alarm upon entry into an area, or other disturbances such as breaking of glass, or the like.